1. Field of the Invention
The present invention relates to semiconducting diamond films with heat-resisting ohmic electrodes applicable for electronic devices which can be used at high temperature, and to fabrication method for such films.
2. Description of the Related Art
Diamond has a high hardness and a high thermal conductivity, an excellent heat resistance and a high stability against radiation and chemicals. Recently, it has become possible to synthesize diamond films by chemical vapor deposition process (hereinafter, referred to as CVD process). Diamond films thus formed are used as a coating material for speaker diaphragm coatings and heat sinks for integrated circuits and the like.
Diamond is insulating in the undoped state; however, it is p-type semiconductor if boron (B) is doped. The semiconducting diamond has a large band gap (about 5.4 eV) so that its semiconducting properties are not deteriorated even at high temperatures of several hundreds .degree.C. Therefore, semiconducting diamond films are expected to be used as a material for electronic devices operative at high temperatures, which is not possible for devices made using traditional semiconducting materials such as Si, SiC, GaAs or the like.
To apply semiconducting diamond films to electronic devices at high temperatures, it is necessary to establish a technology for forming ohmic electrodes with a good adhesion and an excellent heat resistance on the surfaces of semiconducting diamond films. Conventionally, ohmic electrodes have been made of metallic films for semiconducting diamond film electronic devices.
FIG. 5 is a cross sectional view showing an electronic device (thermistor) with the prior art ohmic electrodes (Abstract of the Fifth Diamond Symposium, p. 114, 1991). Here, a diamond film 12 without impurity doping (hereinafter, referred to as undoped diamond film) was selectively formed on a substrate 11. A boron doped diamond film 13 (hereinafter, referred to as B-doped diamond film) was then selectively formed on the undoped diamond film 12. A pair of electrodes 14, made of a bilayer of a Ti film 14a and an Au film 14b, were formed on the doped diamond film 13 in such a manner as to be spaced apart from each other. Here, the Ti film 14a and the Au film 14b had been sequentially deposited on the B-doped diamond film 13, followed by annealing. It was found that a TiC layer, formed at the interface between the B-doped diamond film 13 and the electrodes 14, is responsible to obtain ohmic electrodes with a good adhesion.
It should be noted that Ti has a strong affinity to oxygen, and is easily oxidized in air. In order to prevent the oxidation of the Ti film 14a, the Au film 14b is formed on the Ti film 14a as a protective film.
FIG. 6a is a cross sectional view showing an electronic device (thermistor) having other ohmic electrodes according to the prior art; and FIG. 6b is a partially enlarged view showing the portion of the ohmic electrodes of FIG. 6a (NEW DIAMOND Vol. 5, No. 2, p. 32, 1989). Here, a B-doped diamond film 22 was formed on a Si.sub.3 N.sub.4 substrate 21. A pair of electrodes 23 are formed by sequentially depositing a Ti film 23a, a Mo film 23b and a Au film 23c on the B-doped diamond film 22 in such a manner as to be spaced apart from each other. Further, a protective film 24 made of SiO.sub.2 is provided between the electrodes 23 on the B-doped diamond film 22 in such a manner as to be slightly extended on the electrodes 23. Lead wires 25 made of Ni are connected to the electrodes 23 at the positions not to be covered with the protective film 24.
The above-mentioned semiconducting electronic devices using the prior art of semiconducting diamond films, however, have a disadvantage that they cannot be used at high temperatures. Namely, as shown in FIG. 5, in the Ti-Au bilayer of electrodes 14, Ti diffuses rapidly into Au at a temperature of 500.degree. C., thereby causing an oxidation of Ti at the surface of the Au film 14a. Accordingly, the ohmic electrodes with such a structure as shown in FIG. 5 tend to be oxidized and hence deteriorated at high temperatures.
If a Mo film 23b is provided between the Ti film 23a and the Au film 23c as shown in FIGS. 6(a) and 6(b), it appears to be possible to prevent the mutual diffusion between Ti and Au, hence to solve the above problem. However, the thermal expansion coefficients of Ti, Mo and Au are significantly larger than diamond. Therefore, if the Ti/Mo/Au electrodes are used in an environment in which the temperature rise and drop are frequently repeated, there occurs a problem that the electrodes are cracked or partially peeled off.